The present invention relates, in general, to Radio Frequency (RF) power detectors; and, in particular, to a method and apparatus for suppressing odd-order harmonics responsible for distorting the RF signal spectrum measured at the output of a Power Amplifier (PA).
Without limiting the scope of the invention, its background is described in connection with RF power detectors used in power control loops.
A wireless-communication system, for example, a Global System for Mobile (GSM) communications, is a standardized system that defines a common method of communication between a Mobile Station (MS), such as a mobile phone or a mobile computer, and a Base Transceiver Station (BTS). The GSM system uses a Time Division Multiple Accessing (TDMA) signaling mode to utilize the available channel frequencies. The TDMA signaling mode defines a carrier frequency comprising multiple TDMA channels, each TDMA channel having multiple time slots. Each TDMA channel has a duration of 4.615 ms and each timeslot has a duration of 577 xcexcs. Each time slot carries either speech or control data in a burst form.
The BTS comprises a transceiver comprising a modulator to digitally modulate the carrier frequency with information, such as speech or control data, and a Power Amplifier (PA) to amplify the modulated RF signal. The PA is required to ramp up to a specific power level, output the signal containing information, and to ramp down to a specified power level in a defined amount of time in order to avoid interference with an adjacent time slot. Non-linear components generated by the PA, the transceiver, or any device capable of generating non-linear components affect the ability of the PA to meet operational constraints imposed by the system.
Amplitude Modulation to Amplitude Modulation (AM-AM) and Amplitude Modulation to Phase Modulation (AM-PM) conversion defines amplitude and phase transfer characteristics of the PA. AM-AM and AM-PM describe changes in the output signal amplitude and phase components relative to a change in the input or output signal power. A PA having poor AM-AM and AM-PM conversions will generate non-linear components causing in-band, intermodulation. In-band, intermodulation results in the RF signal spectrum spreading out and rising up, referred to as spectral re-growth. Spectral re-growth may produce high-switching transients that may cause interference with adjacent time slots.
Turning now to FIG. 1, where a block diagram of prior art Power Control Loop (PCL) used in wireless communication systems, such as a GSM system, is illustrated and denoted generally as 10. PCL 10 comprises a Power Amplifier (PA) 12 having a RF input 14 for receiving RF input power Pi and a RF output 16 for supplying an output power Po. RF input power PI defined as a signal having a carrier frequency modulated with information and output power Po defined as the signal amplified by a pre-determined magnitude. PCL 10 comprises a variable attenuator 18 electrically coupled between RF input 14 and PA 12. Variable attenuator 18 comprises a control input 20 for receiving a control voltage Vc.
PCL 10 further comprises a coupler 22 electrically coupled to RF output 16 for coupling a portion of output power Po through a negative feedback path 24. PCL 10 further comprises a RF power detector 26 electrically coupled to coupler 22 for detecting a coupled power Ei. RF power detector 26 comprises a detector input electrically coupled to coupler 22 and a detector output containing a detected voltage VD. Detected voltage VD is defined as a voltage proportional to the envelope of coupled power Ei. RF power detector 26 may comprise, for example, a detector circuit for detecting coupled power Ei, a filtering circuit for filtering the carrier frequency from the detected coupled power EI, and converting circuitry for converting the filtered detected power to a voltage.
PCL 10 further comprises a comparator 28 having an inverting input 30 electrically coupled to the detector output containing detected voltage VD and a non-inverting input 32 containing a supplied reference voltage VR. Supplied-reference voltage VR, supplied by an external source such as a Digital Signal Processor or Applications Specific Integrated Circuit (ASIC), is a correct representation of output power Po. Comparator 28 compares detected voltage VD and reference voltage VR and supplies the difference, an error voltage VE, which is filtered through a loop filter 34. The filtered signal, a control voltage Vc, causes an increase or decrease in the attenuation level correcting any deviation in output power Po.
FIG. 2 illustrates a prior art RF spectrum window denoted generally as 50. RF spectrum window 50 comprises a spectrum mask 52 and a RF signal spectrum 54. Spectrum mask 52 defines the boundaries of RF signal spectrum 54 defined by system requirements, that is, system requirements for a GSM 900 MHz transceiver. RF signal spectrum 54 is a measure of RF power Po at output 16. RF signal spectrum 54 illustrates the signal amplitude and phase-content changes with different power levels as a result of AM-AM and AM-PM conversions. As a result of changes in the signal phase content, RF signal spectrum 54 becomes asymmetrical beginning around 400 kHz offset from center. The negative feedback circuit corrects AM-AM intermodulation; however, AM-PM intermodulation is not corrected.
As may be seen, an improved apparatus to suppress odd-order harmonics generated as a result of AM-PM conversions could improve the linearity of a Power Amplifier (PA).
The present invention presents an improved apparatus for reducing AM-PM intermodulation affecting the RF signal spectrum at the output of a Power Amplifier (PA).
A Power Amplifier (PA exhibits on-linear characteristics as a result of Amplitude Modulation-Phase Modulation (AM-PM) conversion resulting in intermodulation. Inter-modulation affects the RF signal spectrum at the output of the PA. The invention presents a method and apparatus for reducing intermodulation generated at the output of a PA as a result of AM-PM conversion. Presented is a RF power detector utilized in feedback control circuitry where the induction of odd-order harmonics into the feedback path acts to reduce AM-PM conversion.
The RF power detector comprises a detector input for receiving RF power and a detector output containing a pre-corrected, detected voltage. The pre-corrected, detected voltage comprises odd-order harmonics. The RF power detector further comprises a pre-correcting circuit for generating the odd-order harmonics at a specific power level. AM-PM conversions may significantly affect the RF signal spectrum at a specific power level. The pre-correcting circuit may be biased to generate odd-order harmonics at this specific power level.